Lead (Pb) is a heavy metal that is well known for its toxicity to humans. Lead poisoning in humans can occur by skin contact, breathing, eating, or drinking materials that contain lead. Lead containing products such as paint, plumbing, and gasoline have been banned in many countries including the U.S. and countries in the European Union (EU). Children are particularly susceptible to lead poisoning and lead poisoning can cause developmental problems in children, such as learning disabilities, neurological damage, anemia, stunted growth, behavioral disorders, impaired speech development, loss of hearing, renal damage, and hyperactivity, just to name a few. Once lead is introduced into a bio-system, it is almost impossible to get rid of because lead does not dissolve in water, is not biodegradable, cannot be burned off, and does not dissipate or decay in concentration over time.
A primary source of lead poisoning in humans is through groundwater contamination from products or materials that contain lead. For example, landfills often contain electronic products that have been dumped in the landfill. Those electronic products include circuit boards on which components have been soldered using a lead-based solder. The elements tin (Sn) and lead (Pb) are key components in the lead-based solder (e.g. a lead-tin solder) and the lead-tin solder itself is a fundamental material for electrically connecting and joining a component to a circuit board. Eventually, rain and other environmental factors cause the lead (Pb) in the lead-tin solder to leach out into the groundwater.
Although lead-tin solder accounts for a small percentage (<1%) of worldwide lead consumption, the proliferation of electronic products, especially consumer electronics products, that are purchased and then later discarded in landfills, has prompted many countries to enact legislation that bans the use of lead-tin solder. For example, Japan and the EU have pending lead-free legislation that will restrict the use of lead-based solder. The EU Restrictions on Hazardous Substances (RoHS) will take effect on Jul. 1, 2006. These restrictions on lead-based solder will not only impact electronics manufactures in the enacting nations, but also electronics manufactures who trade goods with those nations. In either case, manufactures in the electronics industry are faced with developing an economically viable and environmentally friendly substitute for lead-based solder that complies with the lead-free restrictions.
Currently, a lead-free solder is being considered as a replacement for lead-tin solder. Examples of a composition of the lead-free solder include: (1) 99.3% tin (Sn) and 0.7% copper (Cu); (2) 95.5% tin (Sn), 4.0% silver (Ag), and 0.5% copper (Cu); and (3) 92.3% tin (Sn), 3.4% silver (Ag), 1.0% copper (Cu), and 3.3% bismuth (Bi). However, there are some implementation problems that arise from the use of lead-free solder that must be overcome so that manufacturing of electronics using lead-free solder is economically viable. One such problem is an increased contact failure rate during in-circuit-test (ICT) of vias on PC boards. Typically, a PC board will include several test vias that include a lead-free solder on a pad of the via. The vias are probed by a test fixture after a reflow soldering of components to the PC board. The probing is necessary to ensure that the PC board and the components connected with it are functioning properly. A lead-free solder is easier to probe than a bare finished metal with the exception that vias pose a problem because a flux residue coats the via after reflow and the residue serves as an insulating barrier that prevents electrical contact between a probe and the lead-free solder on a pad of the via.
Turning to FIG. 1a, a prior electronic circuit 300 includes a PC board 301 including components (311, 321) that are soldered 315, using a lead-free solder, to pads 313 that are positioned on a surface 301t of the board 301. The dashed outline depicts a portion under the components (311, 321) where the lead-free solder is reflowed to connect the components (311, 321) to their respective pads 313. The components (311, 321) can be surface mount devices, such as resistors, capacitors, and integrated circuits, for example. Electrically conductive traces 312 connect the components (311, 321) to other elements (not shown) that form the electronic circuit 300. A via 303 is one such element that can be connected with the components (311, 321) by the traces 312. As will be described below, a lead-free solder paste is applied to the vias 303 and the pads 313. The lead-free solder paste includes a lead-free solder component and a flux component. After a reflow soldering process, a pad 302 of the via 303 is covered by a lead-free solder 335s and a hole 305 of the via 303 is filled with the solder 335s. The solder 335s includes a divot that is capped by a flux 335f that is recessed in the divot. The solder 335s also serves to solder the components (311, 321) to their respective pads 313 as denoted by 315.
To facilitate the description and inter-relation between figures, a coordinate system with three axes orthogonal to one another is provided as shown in FIG. 1. The axes intersect mutually at the origin of the coordinate system which is intended to be the center of the via 303. The axes in all figures are offset from their actual locations for clarity of illustration. Moreover, FIGS. 1a˜1c are understood to be a plan view of the electric circuit 300 and PC board 301 (FIG. 1a), via 303 (FIG. 1b) and stencil 330 (FIG. 1c) according to the YX-plane. FIGS. 1d˜1h are cross sectional views of the PC board 301, via 303 and stencil 330 according to the ZX-plane. Similar comparisons may be made for the figures as well.
Reference is now made to FIGS. 1b through 1e, where the via 303 includes a diameter DV and a stencil 330 that is used as a mask to apply the aforementioned lead-free solder paste to the via 303 includes an aperture 331 that has a diameter DS. Typically, as was done when lead-based solder was used to solder components to a PC board, a one-to-one pasting (1:1) of a solder paste to the via was accomplished by using a stencil with an aperture that had a diameter that was greater than or equal to the diameter of the via. Accordingly, in FIG. 1c, the diameter DS of the prior stencil 330 would be greater than or equal to the diameter DV of the via 303 (i.e. DS≧DV) of the via 303 in FIG. 1b. When DS>DV, then the pasting of the pad 302 of the via 303 is referred to as an over pasting or over printing of the via.
Consequently, the diameter DS of the aperture 331 completely covers the pad 302 of the via 303 as depicted in FIG. 1f, where the stencil 330 is positioned in contact with the surface 301t of the board 301 and a lead-free solder paste 335 is applied to the stencil 330 and flows through the aperture 331 and covers the pad 302 of the via 303. Subsequently, heat h is applied to the lead-free solder paste 335 during a reflowing process. The reflowing causes a solder component 335s of the paste 335 to wet and adhere to the pad 302. Additionally, a substantial portion of the solder 335s flows into the hole 305 of the via and a flux component 335f of the paste 335 also flows into the hole 305. As a result, the solder 335s fills up a substantial portion of the hole 305 and the flux 335f pools on top of the solder 335s and plugs the hole 305. The plugging of the hole 305 and the pooling of the flux 335f are due to a higher solids content of the flux 335f used in the lead-free solder paste 335 and to the aforementioned 1:1 pasting of the lead-free solder paste 335 to the via 303.
With respect to the above description, the prior art method of 1:1 pasting may be more fully appreciated with respect to FIGS. 18a˜18e. Specifically, FIGS. 18a˜18e provide a perspective view to interrelate the top pan views of FIGS. 1b and 1c, and the cross sectional views along I-I and II-II of FIGS. 1d and 1e and the cross sectional view of the pasting process presented in FIGS. 1f˜1h. 
More specifically, FIG. 18a illustrates the PC board 301 having via 303 disposed therein such that pad 302 is exposed upon the surface of the board 301 and hole 305 extends into the board 301. As in FIG. 1b, via 303 had diameter DV. In FIG. 18b, stencil 330 is shown disposed upon board 301 and aligned to via 303. As in FIG. 1c, stencil 330 provides aperture 331 having diameter DS, the aperture 331 aligned directly upon via 303. As DS≧DV the pad 302 and hole 305 of via 303 are fully exposed through aperture 331.
FIG. 18c corresponds generally to FIG. 1f, and shows lead free solder paste 335 applied to the stencil 330 being urged into the aperture 331 to cover the pad 302 and hole 305 of via 303. In at least one embodiment, a blade 360 serves to facilitate the uniform flow of paste 335 into the aperture 331. In FIG. 18d, the stencil 330 has been removed. As shown, the lead free solder paste 335 is a continuous mass, i.e. a disc, disposed upon the via 303 and covering pad 302.
Upon the subsequent application of heat h to paste 335 during the reflow process, the solder component 335s of the paste 335 is caused to wet and adhere to the pad 302 (see FIG. 18e). With respect to the size of the disc of paste 335, the surface area of pad 302 is small. As such a substantial portion of the solder 335 will flow into the hole 305 and the flux 335f component will pool on top of the solder 335s within the hole 305. As is shown in FIG. 18e, this results in a contiguous ring of solder 335s upon pad 302 circumferentially about the plugged hole 305 now having a top layer of pooled flux 335f, that may be generally flat, concave or convex. Moreover, the solder 335s sections shown in the cross section view of FIG. 1h are not separate and distinct sections, but correspond to dotted line areas 1800 and 1800′ in FIG. 18e as cross sections of a contiguous ring of solder 335s. 
Because of the 1:1 pasting, more solder paste 335 than is needed is applied to the pad 302. During the reflowing, that excess solder 335s in the paste 335 flows into the hole 305. As a result, the flux 335f pools on top of the excess solder 335s in the hole 305 as depicted in FIGS. 1i and 1j. The flux 335f can be substantially flush with a top surface of the pad 302 or the flux 335f can form a dome 335fc that can be convex in shape (e.g. positioned above the top surface of the pad 302) or concave in shape (e.g. positioned below the top surface of the pad 302). In either case, a tip 350t of a probe 350 that is urged u into contact with the via 303 is not able to make an electrical connection C with the solder 335s on the pad 302 because the tip 350t cannot penetrate far enough into the hole 305 due to the blockage caused by the flux 335f. Consequently, a contact surface 350c of the probe cannot connect with the solder 335s on the pad 302 and an electrical continuity cannot be reliably established between the probe 350 and the via 303. Moreover, the vias 303 are typically coated with an organic solderability preservative (OSP) prior to the applying of the solder paste 335. The OSP is a low cost coating that if left over pasted, prevents repeatable contact with the via 303.
Returning to FIG. 1a, after the reflowing of the solder paste 335, it is desirable to probe the board 301 to perform an in-circuit-test (ICT) to verify proper functioning of the board 301. However, the flux 335f pooling in the hole 305 prevents a reliable ICT using the probe 350 as was depicted in FIG. 1j. Essentially, the probe 350 fails to make a good electrical connection in high percentages with the solder 335s on the pad 302. The inability to repeatedly perform a reliable ICT on the board 301 results in an increased manufacturing cost for re-testing of lead-free solder treated boards, waste cost associated with boards that are mistakenly scrapped, increase liability exposure during a warranty period for products that contain bad boards, a negative impact on brand loyalty caused by defective products, and costs associated with troubleshooting the boards and the ICT functional testers used to probe the boards.
Potential solutions to the above problems include using a higher cost board finish and using test pads instead of test vias. The use of a higher cost finish (e.g. electrolytic Ni/Au) is not acceptable for volume manufacturing, particularly for low margin consumer electronics products. Other notable finishes such as immersion tin and immersion silver may lead to whisker growth and failure mechanisms associated with whisker growth. The use of test pads versus test vias has the disadvantage of using up precious real estate on the PC board and adding capacitance to the signal net that can compromise signal speed, performance, and reliability. An additional solution is to chemically wash the boards; however, washing increase manufacturing costs and the chemicals used create environmental problems of their own.
Consequently, there is a need for a method of treating a via with a lead-free solder that is economically viable and that prevents solder and flux from plugging up a hole of the via. Moreover, there exists a need for a method of treating a via with a lead-free solder that eliminates an over pasting of the via and reduces an amount of a lead-free solder paste that is applied to a pad of the via. Finally, there is also a need for a method of probing a via that has been treated with lead-free solder that provides reliable and repeatable probing of the via during in-circuit-test.